A person's brain has many brain nerve cells, and a magnetoencephalography (hereinafter referred to as “MEG”) signal is generated by ionic electrical activity of the brain nerve cells. If an MEG signal is measured, medical applications such as diagnosis of brain functions, localization of an epilepsy developing location, and cognitive function diagnosis are made possible. However, an MEG signal generated from a brain is a very weak signal of tens to hundreds of femto-Tesla (fT). A high-sensitivity magnetic sensor and technical development capable of effectively shielding earth's magnetic field and environmental noise are required to detect such a weak signal with a high signal-to-noise ratio (SNR).
A superconducting quantum interference device (hereinafter referred to as “SQUID”) sensor using a superconductor is a magnetic sensor having very high sensitivity and is necessarily used in an MEG signal measuring system. A SQUID sensor needs to be connected to a pick-up coil to measure a magnetic signal with the SQUID sensor. According to types of pick-up coils, SQUID sensors are classified into a magnetometer adapted to measure a magnetic field value and a gradiometer adapted to measure spatial differential of a magnetic field.
A method for removing environmental magnetic noise includes a method for fabricating a signal pick-up coil in the form of a gradiometer and a method for mounting a magnetically shielded room (MSR) using a metal having high permeability and a metal having high electrical conductivity. Moreover, the environmental magnetic noise may be additionally removed through the procedure of processing a measured signal.
When a pick-up coil is fabricated in the form of a gradiometer, a first-order gradiometer is generally introduced. In this case, spatially non-uniform noise may not be removed effectively or a reference channel may reduce a signal to cause SNR reduction. Magnetic shielding using a magnetically shielded room may effectively shield a magnetic field but requires a long fabrication time and a wide fabrication space. In addition, since magnetic shielding must use a metal having high electrical conductivity such as Permalloy, much cost is required. To overcome these disadvantages, various studies have been conducted on magnetic shielding using Meissner effect that is a characteristic where a magnetic field cannot penetrate a superconductor under a superconducting state. Superconducting shield has constant shielding performance according to a frequency and is an ideally perfect shielding method.
When a superconductor is implemented in the form of a helmet using superconducting shielding characteristics, a superconducting helmet may suppress a noise from a low frequency to a high frequency due to superconducting shielding effect in the superconducting helmet. In particular, when a conventional magnetically shielded room is used, many high-priced Permalloys must be used to obtain a high shielding factor in a low-frequency region of 0.1 Hz or less. However, in case of superconducting shield, a high shielding factor may be obtained even in the low-frequency region of 0.1 Hz or less.
According to superconducting shielding theory, when a magnetic signal source Msource is disposed at a position spaced apart by a distance “a” in a direction perpendicular to a superconductor plane, current flowing to a superconducting shielding material surface, a virtual magnetic signal source Mimage of the same size but an opposite direction is likely to exist opposite to the magnetic signal source Msource. Therefore, theoretically, a gradiometer spaced part from a superconductor surface by the distance “a” operates the same as a primary gradiometer whose base line is “2a”, which was proved by the Los Alamos National Laboratory (LANL) study group.
Thus, when a superconductor is fabricated in the form of a helmet, magnetic shielding may be achieved in a superconducting helmet. According to depth of a signal source desired to be measured, spaced distance between a superconductor material surface and a pick-up coil may be adjusted to determine length of a base line. In addition, the superconducting shield may provide a constant shielding effect according to a frequency. The LANL study group announced the effectiveness of superconducting shield by manufacturing an MEG apparatus in the form of a shielding helmet directly cooled with liquid helium and measuring a shielding factor depending on each position of a gradiometer in the helmet and a somatosensory signal.
However, according to a result of the LANL study group, a signal-to-noise ratio of a gradiometer disposed at the edge of an MEG helmet was lower than when superconducting shielding is not performed. The reduction in the signal-to-noise ratio of the gradiometer disposed at the edge of the MEG helmet is caused by the fact that density of a magnetic-force line increased at the edge of the helmet. An MEG signal was actually measured depending on whether superconducting shielding is performed. When the superconducting shielding was performed, a somatosensory signal near a vertex was measured to have a high signal-to-noise ratio whereas an evoked signal for an auditory cortex and a visual cortex reacting at left and right temporal regions and an occipital region had a very low signal-to-noise ratio. In particular, when superconducting shielding was performed, a cardiac magnetic signal and an interest vibration noise of a measurement person were measured to be very high and great at the edge of a superconducting shielding helmet. The significant increase in external noise intensity is caused by magnetic field focusing effect at the edge of the superconducting shielding helmet.
Referring to U.S. Pat. No. 7,729,740, to overcome the above problem, the LANL study group mounted a reference magnetometer for measuring only an environmental magnetic noise outside a superconducting shielding helmet and applied an adaptive filter to remove the noise. However, when the adaptive filter is used, the inside of the superconducting shield and an external nose must have the same frequency and the same frequency element. In addition, when the noise element is much greater than a signal element desired to be measured, the application of the adaptive filter is not effective. In particular, a magnetic signal generated from a person's heart is detected by a magnetometer inside the helmet but is not often detected by a reference magnetometer. Accordingly, there is a need for a novel superconducting shielding structure to improve a shielding effect at the edge of a helmet.